Semiconductor nanocrystals are widely used in the study of biochemical and biological systems. Semiconductor nanocrystals can be excited over a wide spectrum of wavelengths and display intense luminescent emission over an extremely narrow bandwidth that is dependent on nanocrystal size and composition. Nanocrystals can also be made insensitive to their medium or environment. These properties allow semiconductor nanocrystals to be used as ultrasensitive luminescent reporters of biological states and processes in many systems.
Semiconductor nanocrystals may be made using techniques known in the art. These methods typically produce nanocrystals having a coating of hydrophobic ligands, such as on their surfaces. Because the surface of the nanocrystal has many binding sites for such ligands, the typical process results in coating of the exposed surface of the nanocrystal with a layer of alkyl groups at the outer surface, and produces a nanocrystal with a surface that is hydrophobic, i.e., incompatible with water.
High temperature pyrolysis gives the synthetic chemist a substantial degree of control over the size of the particles prepared. One disadvantage of this method, however, is that the particles are sequestered in reverse micelles of coordinated, hydrophobic coating of surfactant molecules, such as, for example, trioctyl phosphine (TOP), trioctyl phosphine oxide (TOPO), or tetradecylphosphonic acid (TDPA). While this surfactant layer helps to protect and stabilize the nanocrystal from rapid degradation, they are only dispersible in organic solvents such as chloroform, dichloromethane, hexane, toluene, and pyridine. This is problematic insofar as many applications rely on the fluorescence emission of the semiconductor nanocrystals and require that the nanocrystals be water soluble or water dispersible. In particular, for biological applications, nanocrystals that are soluble or dispersible in water are desirable. Therefore, it may be necessary to make the surface of the nanocrystal, which is typically coated with hydrophobic ligands, compatible with water or biological media.
Although some methods for rendering semiconductor nanocrystals water dispersible have been reported, they are problematic insofar as the treated semiconductor nanocrystals suffer from significant disadvantages that limit their wide applicability. For example, a Cd(OH)2-capped CdS sol exhibits photoluminescent properties that are pH dependent. The sol could be prepared only in a very narrow pH range (pH 8-10) and exhibited a narrow fluorescence band only at a pH of greater than 10. Such pH dependency greatly limits the usefulness of the material; in particular, it is not appropriate for use in biological systems. Other groups have replaced the organic passivating layer of the semiconductor nanocrystal with water-soluble moieties; however, the resultant derivatized semiconductor nanocrystals often exhibit a loss in luminescence. For example, short chain thiols such as 2-mercaptoethanol and 1-thio-glycerol have been used as stabilizers in the preparation of water-soluble CdTe nanocrystals, but the resulting coated semiconductor nanocrystals were not stable and photoluminescent properties degraded with time. Other more exotic capping compounds such as deoxyribonucleic acid (DNA) have been reported with similar results. In addition, subjecting coated nanoparticles to ligand exchange typically results in nanoparticles having a lower quantum yield and lower colloidal stability than the coated nanoparticles prior to ligand exchange, and incomplete ligand exchange can cause batch-to-batch variability.